[llvm-commits] [llvm] r66922 - in /llvm/trunk: lib/Target/X86/X86InstrInfo.td test/CodeGen/X86/tls13.ll test/CodeGen/X86/tls14.ll

[Rafael Espindola]

> I agree with your analysis. It looks like too much complexity to handle
> in Select. This is something X86ISelLowering should handle.
>
> I think Chris' interest is to have a single place where TLS is lowered
> to segment register usage, with the particular place being less
> important. This can be accomplished with the approach you describe
> only with THREAD_POINTER renamed to GET_GS, so that it's clear that
> Lowering has fully lowered the TLS addressing.

The problem is that gs is not the same as THREAD_POINTER. The content
in gs:0 is.

The attached patch is correct, but has some generated code performance
regressions. What it does is

*) LowerToTLSExecModel lowers a thread local address to (add
THREAD_POINTER, offset) (for local exec)
*) THREAD_POINTER is selected to mov %gs:0,

The problem is that this produces

movl	%gs:0, %eax
movl	i at NTPOFF(%eax), %eax

Instead of

movl	%gs:i at NTPOFF, %eax}

I can try to fold the instructions, but the optimization is
only valid because the GNU TLS model requires
that the value in gs:0 be its own address.

The other option is to in LowerToTLSExecModel produce a
SegmentAddress node. The problem now is when we just
want to compute the address, not load from it. Since LEA
doesn't use segments, we would have to select
SegmentAddress(address, GS) into

mov gs:0, eax
leal address(%eax), %eax

This again depends on the GNU TLS model. In both cases
(using THREAD_POINTER or SegmentAddress) both
lowering and selection have to be aware of the TLS model.

If using SegmentAddress, another interesting case will be

int __thread a;
int __thread b;
void f(int **c, int **d)
{
  *c = &a;
  *d = &b;
}

In this case we will select

mov %gs:0, eax
leal address_a(%eax), %eax
....
mov %gs:0, eax
leal address_b(%eax), %eax

Will we automatically remove the second
"mov %gs:0, %eax" and use the value from the first one?

So what strategy do you suggest?

1) Remove THREAD_POINTER and add a SegmentAddress
2) Keep THREAD_POINTER and try to figure out how to fold
 "mov gs:0, eax" into (for example)
 "movl i at NTPOFF(%eax), %eax"?

> Dan

Thanks,
-- 
Rafael Avila de Espindola

Google | Gordon House | Barrow Street | Dublin 4 | Ireland
Registered in Dublin, Ireland | Registration Number: 368047
-------------- next part --------------
A non-text attachment was scrubbed...
Name: llvm-wip.patch
Type: text/x-diff
Size: 36646 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20090402/a4f8062b/attachment.patch>